The predominant source of ketone body carbons under all physiological circumstances is believed to be fatty acids. However, ketogenic amino acids can also give rise to ketone bodies but their contribution has been dismissed as being inconsequential. The problem with this conclusion is that there is no quantitative information to either support or contradict it. We have obtained in vivo data in starved rats showing that one third of ketone body carbon is derived from sources other than fatty acids. Preliminary data suggested leucine may be a major component of this missing carbon source. We propose to quantify the fraction of ketone body carbon derived from leucine under different physiological circumstances in which the rate of ketogenesis varies. We also propose to obtain preliminary information on the conversion of other ketogenic amino acids to ketone bodies.